1. Field of the Invention
The present invention relates to a fluid mixing system and a fluid mixing apparatus for mixing and delivering a plurality of fluids at a predetermined mixture ratio.
2. Description of Related Art
In a semiconductor manufacturing process and the like, after an insulating film or metal film is deposited on a wafer (a film formation step), a photoresist pattern is formed on the wafer (a photolithographic step), then the film is processed using the photoresist pattern (an etching step), then a conductive layer is formed on the silicon wafer (an impurity doping step) and the uneven film surface is smoothed by polishing (a CMP step). Meanwhile, in the semiconductor manufacturing process, dirt or dust is removed from the wafer (a cleaning step), the used photoresist is removed (a resist peeling step) and the wafer is heated (an annealing step).
As above, in a semiconductor manufacturing process, different kinds of steps are combined and carried out repeatedly in order to make devices such as transistors and wiring in a wafer. In the process, at steps such as thin film formation, annealing and etching steps, a plurality of gases are mixed and supplied to the wafer. At the photolithographic step or the like, a plurality of chemical liquids are mixed and supplied to the wafer. Since the mixture ratio of gases or the mixture ratio of chemical liquids influences the film thickness or the like, it must be strictly controlled. For controlling such mixture ratios, a semiconductor manufacturing system conventionally uses a fluid mixing system which mixes and delivers a plurality of fluids (gases, chemical liquids, etc).
FIG. 14 is a block diagram showing an example of a conventional fluid mixing system 100.
The fluid mixing system 100 is arranged to mix two kinds of gasses X and Y supplied from gas sources 103A and 103B respectively at a specified mixture ratio and deliver the mixed gas to a process chamber 111 which is depressurized by a vacuum pump 112.
In the fluid mixing system 100, when a first valve 102A and a second valve 102B are opened to allow the gasses X and Y to flow into the process chamber 111, a first mass flow controller 101A and a second mass flow controller 101B adjust the gasses X and Y to preset flow rates depending on the mixture ratio. The gasses X and Y delivered from the first and second valves 102A and 102B converge and mix before flowing into the process chamber 111.
As the two kinds of gasses X and Y mixed at a predetermined ratio are supplied, processing such as film formation (deposition) is started in the process chamber 111 (for example, see JP2007-175691A).
However, in the conventional fluid mixing system 100, it takes time from when the first and second mass flow controllers 101A and 101B start flow control until the gases X and Y are adjusted to the preset flow rates and are stably supplied to the process chamber 111.
Concretely, the inventors conducted an experiment where they specified 6 sccm as the flow rate for the gas X with a larger specific gravity (for example, SF6) and 200 sccm as the flow rate for the gas Y with a smaller specific gravity (for example, nitrogen) to obtain a mixture ratio of 3:100 and measured the output flow rates from the first and second mass flow controllers 101A and 101B and the flow rate at an outlet of the fluid mixing system 100 delivering the mixed gas to the process chamber 110. The flow velocity depends on a pipe diameter. In this experiment, the above gasses X and Y are identical in terms of flow velocity conditions which are associated with pipe diameters. The result of the experiment is shown in FIG. 15.
As indicated by a first MFC command signal and a second MFC command signal in FIG. 15, in the fluid mixing system 100, as the first and second mass flow controllers 101A and 101B are turned ON to start controlling the above gasses X and Y simultaneously, the output flow rates from the first and second mass flow controllers 101A and 101B reach the respective preset flow rates about one second after the start of flow control, as indicated by a solid line and a dotted line in FIG. 15.
On the other hand, as indicated by a bold line in FIG. 15, the outlet flow rate of the fluid mixing system 100 becomes stable at 200 sccm about five seconds after the start of flow control and then about 15 seconds after the start of flow control, it begins to increase gradually and about 40 seconds after the start of flow control, reaches 206 sccm, a total flow rate of the above gases X and Y, and stabilizes thereat.
As described above, in the fluid mixing system 100, even after the first and second mass flow controllers 101A and 101B turn ON simultaneously and start controlling the gases X and Y to the preset flow rates, the gas X having a larger specific gravity (heavier gas) reaches the process chamber 111 later than the second gas Y having a smaller specific gravity (lighter gas). It takes as much as about 40 seconds until the gases X and Y reach the process chamber 111 stably at the respective preset flow rates.
The inventors studied the above reason and reached the following conclusion.
The gas Y, smaller in specific gravity than the gas X, is easier to flow than the gas X. Besides, the flow rate of the gas Y is higher than that of the gas X. Therefore, the gas Y generates a larger differential pressure between the second mass flow controller 101B and the process chamber 111 and thus the gas Y is likely to be supplied to the process chamber 111 at the preset flow rate earlier than the gas X.
On the other hand, the specific gravity of the gas X is larger than that of the gas Y and the gas X is less easy to flow than the gas Y. When the gas X is going to join the gas Y, the pressure in the process chamber 111 has already risen due to the gas Y, which is a more difficult condition for the gas X to flow than for the gas Y. In short, it is not easy for the gas X to join the gas Y. It is not until the pressure of the gas X delivered from the first mass flow controller 101A becomes higher than the pressure in the process chamber 111 that the gas X starts to join the gas Y. Then, the gas X is gradually increased in flow rate and allowed to be supplied to the process chamber 111 at the preset flow rate.
It can be thought as above that the lighter gas Y retards flow of the heavier gas X and the gas X reaches the process chamber 111 later than the gas Y.
At steps in the semiconductor manufacturing process in which a mixed gas is used, processing is started after the mixture ratio of the mixed gas is stabilized, that is, after the flow rates of plural gases to be mixed become stable at their respective preset flow rates. This waiting period is considered waste of time because no processing is done on the wafer during that period, which leads to a decline in the productivity in the manufacture of semiconductors. With this background, in the semiconductor manufacturing industry and others, there has been a strong demand for a system in which the flow rates of fluids to be mixed are stabilized quickly.